FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

The Dog Days of Summer

As I write this we are riding a string of really hot, really humid, miserable days. These are the dog days of summer, as the saying goes. But did you know that the term “dog day” comes from the realm of astronomy?

And that it has to do with a star that’s not even in the night sky during this time of the year.

Normally the star Sirius is known as a winter/spring star. It is the brightest star in the night sky then. Only over eight light years from earth and monstrously hot to boot, it is one shiny object!

Its name is probably derived from the Greek “seiros,” which means searing. Other peoples gave her similar sounding names. The Celts called her Syr, the Greeks Osiris, and the Egyptians Cahen Sihor.

It’s the Egyptians that have one of the most revered roles for Sirius. But first a little astronomy!

Earth travels around the sun during the year. Our “daytime” is when the side we live on faces the sun. Our “nighttime” is when we are facing away from it. We all learn that in elementary school. But think about that.

Traveling a giant circle all year long means that when we are facing away from the sun - the “center” of our orbit - we are slowly exposing ourselves to new celestial real estate outside our orbit, during the night. The stars we see when we are on this side of the sun are not the stars we see when we are on that side over there months from now, or way over there six months from now.

We see Sirius in our night sky during winter and spring. But where is she now? She is on the daylight side! Unfortunately, the sun brightens the entire sky when it is up so it gets real hard to see the thousands of stars up there during the daytime. But we can cheat a little!

Sirius, being near the sun during this time of the year, will rise with it. The skies are still dark enough during early morning to catch glimpses of stars nearby the sun before full-on daylight kicks in. This phenomenon, when a star rises with the sun, has the fancy title of heliacal rising. And it was of high importance to the Egyptians.

The Nile River meant life to the people of Egypt. And its annual flooding during this time of the year was highly desired because it essentially fertilized all the land around it.

The flooding came about the same time each year, and you’ll never guess what star rose with the sun at that time. Sirius, of course! And Sirius was given the credit for the renewing of the land.

So the annual appearance of Sirius with the sun came with great hope and anticipation. The Egyptians so valued this time of regeneration that they set their calendars by it.

Dr. E. C. Krupp writes in his book Beyond the Blue Horizon, “Egypt, it was said, is a gift of the Nile, but the Nile, as far as the Egyptians were concerned, was a gift of Sirius.”

And the star played other important roles in Egyptian culture, both in their creation myths and in their myths of living and dying and being reborn.

But the Greeks and Romans had a special place for Sirius, as well.

It was in their mythical stories part of the constellation Canis Major, one the dog constellations near Orion. Sirius itself was known as the Dog Star. And it’s here where we finally make the dog connection.

Think of the Mediterranean region during the summer. It’s hot, it’s oppressive. A good time is not had by all. And during the days of the ancients hot, humid weather was not exactly a form of disease control.

And, of course, the hotter weather and all the travail that accompanied it must be due to that searing hot star that rose with the sun - that Dog Star!

Hence, the weeks around which Sirius rises with the sun are the “dies caniculares,” the dog days.

If you are an early riser you can see Sirius rise with the sun. Now understand that rising at the same time as the sun doesn’t necessarily mean at the same place. For us here in southern California, you’ll have to look to the south about 45 degrees, and there she’ll be rising above the horizon just before the sun.

After you take part in this ancient rite of observing the heliacal rising of the Dog Star, start brewing the iced tea, shut the windows, dress lightly, and prepare yourself for what’s likely to be another dog day.

Our Very Unique Skies

Looking up in the night sky, away from city lights, we see myriads of stars. So did our ancestors. The mystery and majesty of the starry night impressed them then as it does many of us now. Most were driven to put sacred stories to the stars, some were tempted to worship them.

But the heavenly arrangement above is unique to our time in the history of the universe, and to our place in this galaxy of ours. The celestial painting above is ours and ours alone.

People for thousands of years assumed, understandably, that the stars above were fixed, unmoving. Many believed they were stuck somehow on a colossal sphere enveloping earth at some finite distance. The whole sphere moved around us giving the stars the appearance that they were moving in unison over our heads during the night.

We know now that they are not fixed points of light, but thousands of fusion reactors spread all about us at greatly varying – and inconceivably large - distances.

If we were to travel out into space we’d see that the constellations appear relatively the same – for a while. But moving out at greater distances, far beyond the edge of the solar system, the stars in the constellations would appear to shift about. Before we knew it, nothing in the heavens would be recognizable as any of the starry designs we see here from earth.

It’s pretty straightforward why. If you saw a forest at a distance, the trees would seem to trace out a certain pattern. Walking a little to the right or left essentially makes no difference to the pattern of those distant trees.

But walk toward the forest and the closer you get the more the trees seem to change position. The pattern you saw from a distance is no longer there. Get to the forest itself and walk in amongst the trees and whole new patterns show up continuously.

But the constellations don’t change only because of our location, they change with time, as well.

The stars we see in our skies are not the same stars the dinosaurs saw in theirs. What humans have seen above in the last tens of thousands of years is unique because of the time we are living.

One reason is that stars really do move. And some are moving pretty darn fast. But they are so unbelievably far away that it takes a long, long, long time to notice any movement at all through the background of other stars.

But give them enough time and the stars will move about, changing the starry designs as they do.

Another reason our view is unique has to do with this fact: The stars that we can actually see are the brightest available stars up there. They are the big burners, the monster stars. There’s not a tiny dim one in sight. Those little guys are numerous, to be sure, but not visible.

The stars that do light up our celestial sphere are short-lived. By their very nature they are doomed to die soon. They burn themselves up in just tens of millions of years, not tens of billions. That means that most stars alive now weren’t even born when dinosaurs ruled the earth, about 65 – 225 million years ago.

Moreover, stars visible to the dinosaurs back then were similar to ours in composition, so those ancient stars have since faded away or exploded.

T. Rex didn’t see our stars; we don’t see his.

And the stars we see in the heavens now will not be here tens of millions of years down the line. The majority of them are destined to die out by then.

New stars are being formed, to be sure, ensuring another generation of future constellations. But that scene will be different in a second profound way. There is less and less available material to build stars, therefore there will be fewer of them to light the sky. The future bodes poorly for those who love a sky full of lights.

Bottom line: All the familiarity of the night sky is familiar only to humans. Before we were here and after we are gone, and anywhere else in the galaxy, let alone the universe, a whole different sky fills the view.

Deep Impact on Understanding Comets

If all goes according to some very intricate planning, the first fireworks celebrating Independence Day will actually happen late night on Sunday the 3rd, at about 11 PM our time. It will happen about 83 million miles away, and you won’t be able to see it unless you have access to a telescope. But it will be big!

Here’s what’s up.

We all know that there are objects in the solar system besides our familiar sun and planets. For example, we have the asteroids, b-zillions of them littering the place. Most are between Mars and Jupiter, but some pass nearby and pose real threats to us.

But we also have our comet visitors, those long hairs from outer space. Far from being the portents of doom that they were to many of our ancestors we now know some things about them – but not everything, by far.

We know from where they probably come, we know how the tails form, we know basically what they are made of – well… kinda sorta. Their full chemistry and how they hold together remain mysteries. And knowing these objects well, these fossil remnants of the creation of the solar system, can help us understand how the whole place was put together more than 4.6 billion years ago.

But to know more about comet innards we have to throw something at it and break it. It’s a common thing in science, this sort of invasive surgery. Physicists throw subatomic particles at each other to see what they are made of; geologists crack open geodes to reveal the beauty within. Now astronomers will throw one fast moving projectile at a comet, Comet 9P/Tempel 1, to reveal the secrets within.

Twenty-four hours before the fireworks begin, NASA’s Deep Impact spacecraft, launched last January and headed for Temple 1 at a breakneck speed, will toss off a washer/dryer-sized projectile towards the 8-mile-across comet. This tiny projectile, weighing in at 820 pounds, will be traveling so fast, 23,000 miles an hour, about 6 miles per second, that when it hits it will be like lighting up 5 tons of TNT. No small explosion that.

The engineers working on it estimate that this tiny projectile will punch a crater into the comet bigger than a football field and about 50 meters deep.

Meanwhile, the main spacecraft will zip by the comet as it happens, recording the whole event from a safe distance.

So what? How can slamming something into a comet tell us its composition?

By studying the impact, scientists can estimate how hard the surface is. Is the comet layered through like earth, with a crust and core? Or is it a thrown together mess? Seeing the size of the impact and the depth of the crater will help these investigators figure that one out.

How far the blast debris (the “ejecta”) gets thrown depends on the gravity of the comet. So observing where the ejecta settle can help estimate density and possibly what stuff is deep down inside.

Analyzing the spectra of the light reflected by all the fireworks will help astronomers figure out what compounds make up the surface of old Temple 1.

And of course the images, close-up and finely detailed, will help determine perhaps how she’s put together at the surface.

But understand that no comets will be harmed in this experiment. One engineer compares the collision to a gnat striking the windshield of a Boeing 747. Only one fatality there.

We on the West Coast might – key word: might – be able to see it when it happens. It’s always a tough call predicting what might happen – there are so many variables – but some scientists predict that the impact might be bright enough to be seen with a backyard telescope. That’s worth breaking out the scope for backyard astronomers like myself.

If interested in the details of when, where, and how, see http://deepimpact.jpl.nasa.gov.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me